The project studies comparative differential fertility and child mortality in four Central American nations (Costa Rica, El Salvador, Guatemala, and Honduras). Attention is focused on the period surrounding and prior to the 1970-round censuses, and particular interest is paid to the factors operating at the environmental level (involving the structure of the community, government and private services, housing, and household characteristics) and at the individual level. Two main sources of data are used: (a) large micro-samples of the 1970-round censuses; and (b) data on small areal units constructed from various sources.